Tourmaline is one of my favorite materials.
I love the colors, the luster, the RI, the texture when it’s cutting, and the ease of polishing. And, I love the strong dichroism – sometimes, even the “closed-C” specimens. I have created a number of designs especially for presenting dichroism, and for maximizing the economic return from closed-C stones.
Following are some of the secrets of presenting Tourmaline using traditional flat-faceting technology, along with some designs you will want to use, as well as some you just want to be aware of.
One of the most common questions from newer faceters is about how to deal with “Closed C” Tourmaline – a uniaxial, anisotropic gem typically showing strong dichroism.
Anisotropy refers to differences in the binding forces between the atoms forming a crystal, according to their orientation within the crystal structure. This feature of physics affects the propagation of light through the material, giving rise to the feature of double-refraction.
Double-refraction is the splitting of one ray of light, as it enters the material into multiple rays, each of which is polarized differently (vibrating in different directions). The split rays travel through the material at different velocities, and may experience differences in selective absorption – causing them to present different colors upon exiting the material. This is pleochroism. In uniaxial gem materials, we often use the term “dichroic”, for two colors.
Here’s a sample that’s Bluish Green on the A/B axis, and Strongly Yellowish Green on the C axis. In Tourmaline, the differences between the color axes can be very strong, often even presenting a cool hue on one axis and a warm hue on the other!
Anisotropic materials contain either one or two pathways along which double refraction is not observed. These are called “optic axes”. Gemstones belonging to the tetragonal, hexagonal and trigonal systems are “uniaxial” because they have only one such optic axis, so they they split light into two total rays. Gemstones belonging to orthorhombic, monoclinic and triclinic systems are called “biaxial” because they have two optic axes, so they split light into three total rays.
Tourmaline is a trigonal system crystal, so it’s uniaxial (one optic axis). In uniaxial crystals the optic axis is called the “C” axis – which, in Tourmaline runs along the longitudinal direction of the usually-pencil-shaped crystals. The C axis in this graphic is indicated by the brown arrow. The other axes are referred to in the singular as the “A/B axis”. This is because light behaves the same along any path through them. Any direction perpendicular to the C axis, as shown by the green disc cutting across the crystal, is on the A/B axis. Due to this feature, Tourmaline can deliver different colors based on the path light travels (relative to the crystal structure) through your finished gem.
This is important to us because in colored gemstones, the quality of color accounts for 60%+ of valuation. In the Faceting Academy paradigm, our primary focus is maximizing economic return on our rough investment. So, keeping that 60% valuation factor in mind, we don’t make much distinction between a “closed” or completely black C axis, a very-dark C axis, and a C axis that’s an undesirable color. For practical purposes in maximizing value, project planning, and design selection these things are all the same.
Starting with the closed-C Indicolite rough from Afghanistan shown in the above photo, we applied “best practice” rules in choosing and placing design elements to produce this result (polished on the Tin+ polishing lap using my Voodoo Magic 5ok polish):
We modified a published design to achieve “best practices” for this gem. The design we started with – and the step-by-step process we used to modify it for closed-C Tourmaline – are in the lower part of this article (members area).
SHOW THE PRETTY WHILE HIDING THE UGLY
Working with pleochroic materials requires a bit of applied crystallography. We also need to remember that pavilion facets are like mirrors, angled to let us look SIDEWAYS from the table:
The left half of the image shows the face-up presentation of a SINGLE needle inclusion, centered in the gem. The right half of the image shows a profile view, illustrating how the pavilion mirror facets give us a side-view of the needle from various positions around it.
When we cut across a Tourmaline crystal, tabling a brilliant-style gem on the A/B axis, our pavilion mirror facets will return views from various directions through the crystal. Some will look across the A/B; some will look down the C; and some will be somewhere in-between, as shown in the eight pavilion mains in this diagram:
Pavilion facets that reflect a view across the A/B will show that color (teal in the schematic). Pavilion facets that reflect a view down the C will show that color (orange in this schematic). And, facets that are in-between will show a mixture of A/B and C colors. For reasons beyond the scope of this article, the C-axis color will be stronger than the A/B. (Imagine mixing different-colored paints – and imagine we’re adding more of the C-axis color than the A/B color.)
Here’s photo of a gem cut in this orientation:
Note the attractive A/B axis color is only pure where pavilion facets are reflecting a view straight across the A/B axis. Examine the photo closely, beginning at the 6 o’clock position and working your way around the gem. Notice how quickly a slight change of direction through the crystal causes the dark and yellow C-axis color to intrude. Facets reflecting light from straight down the C axis display only C-axis color, excluding AB color entirely.
I cut this stone a long time ago, before I understood how to control uniaxial pleochroism. And, this was obviously not a “best practices” design for this piece of rough. (If you have a rough with a pretty C, and you would like to use this design, the download link is in the second half of this article. Do not use this design on an ugly-C Tourmaline unless you just want to duplicate the above effect for academic reasons.)
Here’s a four-view diagram of a standard step-cut design, enhanced with ray traces and theoretical light pattern to illustrate a green A/B axis and a brown C axis:
Notice the percentage of the face-up stone showing the C-axis color? And, if we had a closed-C rough, the areas shown in brown above would be black…
If we use a standard step cut design, we would use-up time and labor creating the pavilion step cuts on the ends of the design – just to increase the area reflecting a color we don’t want. If we instead chopped the ends of the pavilion, using a single facet running from keel to girdle at 70 degrees, we would extinguish reflection of light traveling along the C axis, neutralizing the ugly-C color to a narrow black wedge, or minimizing the view of a black-C:
The volume calculation for the standard step design in a 2:1 L/W is .736, and for the second design is .785 – for a difference of .049. By changing from the standard step cut to the chopped-end cut, we not only enhance our color presentation, we also preserve 6.6% additional weight in the finished gem – and by doing less labor…
A Tourmaline with a desirable C axis color, especially one that matches the A/B nicely, is suitable for cutting in standard Emerald design – or any other design that pumps light through the C axis. Here’s an example. Note the large triangles created by pavilion facets on each end, displaying C axis color.
If you want diagrams for the above designs to play with you can download a PDF of the basic pencil cut here. And, you can download a GemCAD file of the basic pencil cut here.
If you want more technical detail on this topic, the site members’ version of this article includes:
- More basics of moving light in strongly-dichroic uniaxial materials.
- Diagrams showing the PERCENTAGE of C axis color we view relative to the indexes of pavilion facets.
- Additional ray-tracing, explanations, diagrams, and photos.
- Why NOT to use the design variously called Smithsonian Bar, Vertical Bar, or Opposed Bar that’s often wrongly used for closed-C Tourmaline, even by people who should know better. How that design causes extinction and delivers a lower yield than “best practices”.
- TWO additional faceting designs with more life and efficiency, including the “Inverted Bars for Closed-C” design used to cut the closed-C Indicolite at the top of this article.
- Some secrets of DESIGNING faceting plans for Tourmaline.
- TWO links to other faceting diagrams for study and enjoyment.
- Step-by-step how-to for selecting and modifying designs to maximize your return from your own ugly-C Tourmaline (we all have some).
- THREE important SECRETS of shopping Tourmaline rough!